The use of rheological, viscoelastic and dynamic-mechanical measurements to determine the macroscopic properties of materials is a well known art. Several commercially available instruments can make these types of measurements. Kramer et al, incorporated herein by reference, discloses an example of such instruments in U.S. Pat. No. 5,269,190. Kramer discloses variations on the use of a mechanical frame together with transducers for generating mechanical displacement, measuring mechanical displacement, and for measuring force. Ramp functions, in which strain or shear varies as a linear function of time, can be used. Force, as a function of time, may be measured after a sudden change of strain or shear. A manually operated eccentric for the purpose of generating a sudden change in shear or strain is disclosed by Kramer. In both of these cases, and in much of the practice of rheology, such forces can be applied to a sample once while the resulting force as a function of time is recorded. However, these forces are not modulated as a continuous functions of time. The technique of dynamic mechanical analysis does use single-frequency, continuous sinusoidal modulation in the measurement of loss angle as a function of frequency.
Spectroscopic measurements of submolecular components during sample deformation has been called spectro-rheology [R. A. Palmer, C. J. Manning, J. L. Chao, I. Noda, A. E. Dowrey, and C. Marcott, Appl. Spectrosc. 45, 12 (1991), "Application of Step-Scan Interferometry to Two-Dimensional Fourier Transform Infrared (2D FT-IR) Correlation Spectroscopy" and references therein.] However, there has been no use of continuous multifrequency waveforms in such measurements. Instead, single-frequency waveforms have been used almost exclusively. Continuous single-frequency sinusoidal deformation waveforms can be applied to samples to obtain spectroscopic measurements of samples undergoing deformation [see Palmer, et al.] Optical or spectroscopic measurements, particularly of the polarized infrared absorption or polarized Raman scattering of samples, may be used to obtain information about the time dependence of reorientation of the individual submolecular components. The origin of macroscopic rheological properties is the microscopic reorientation of the various submolecular components of a material. Hence, the use of spectroscopic measurements together with various deformation waveforms has the potential to provide deeper insight into the properties of materials.
To date, predominantly two types of deformations have been used in such spectro-rheological measurements. The first is to ramp strain as a single event in which the strain is increased linearly as a function of time. This approach closely follows the art of rheology and has the advantage that it can be used to study samples undergoing very large deformations which are inherently irreversible, i.e., the sample is irreversibly altered. A significant disadvantage of this approach is that it can be done only once with any particular sample. It is usually restricted to slow events because the signal-to-noise ratio (SNR) of measurements made by infrared, Raman and many other spectroscopic techniques is generally too low to observe rapid transients from a single event. The second common approach to spectro-rheological measurements has been to apply a continuously varying, sinusoidal strain having a small amplitude. This technique follows the art of dynamic mechanical analysis. Continuous signals modulate the spectroscopic properties of the sample. This approach has the advantage that the deformation cycle may be repeated many times so that signal averaging may then be used to improve the SNR of the resulting small spectral variations. The effective time resolution may also be greatly increased by such signal averaging. The principal disadvantage of this approach is that the signal amplitude is intrinsically limited by the requirement that the sample not be irreversibly altered by the deformation cycle.
Many of the advantages of rheo-optical spectroscopy have been discussed by Noda [see for example, I. Noda, Appl. Spectrosc. 44, 550 (1990), "Two-Dimensional Infrared (2D IR) Spectroscopy: Theory and Applications".] The general method allows the measurement of the reorientation of submolecular components of a polymer or other sample material. It can simplify interpretation of the spectrum of a material by reducing the portion giving a signature to only those which respond to a particular external perturbation, and further simplifies interpretation by the fact that, in general, each portion of a molecule which does respond to the perturbation does so with a different time delay. Typically the different submolecular groups (not to be confused with the monomer units) of a polymer or other material have differing vibrational absorption frequencies which can be independently monitored with an infrared, Raman or other spectrometer. Hence, the reorientation during or following mechanical perturbation can be observed separately for each subgroup at an appropriate wavelength or energy. Traditionally, these measurements have been made by the use of a single-frequency sinusoidal mechanical perturbation of the sample under study. It is possible, however, to gain advantage in signal-to-noise ratio and/or reduction of measurement time by the use of a multifrequency mechanical perturbation as disclosed herein.
Two simultaneously applied sinusoidal components have been used as a deformation force for spectro-rheology [C. J. Manning and P. R. Griffiths, 9th International Conference on Fourier Transform Spectroscopy, August, 1993, (SPIE Proceedings, Vol. 2089), 248 (1993)]. This approach has the advantage that the frequencies of the two components may be independently optimized for the purpose of obtaining information about particular submolecular motions. However, this approach does not automatically accomplish the necessary optimization of spectral components.
The present invention uses more than two, and preferably many more, frequency components in a deformation waveform. In general, submolecular motions have different time-scales for which different deformation frequencies are appropriate for probing. It is a feature of the present invention to provide a way to stress a sample simultaneously at more than one frequency. Measuring more than one frequency simultaneously can produce a multiplex advantage. It is possible to gain considerable advantage in signal-to-noise ratio and/or reduction of measurement time by the use of a multifrequency mechanical perturbation. The advantage is equal to square root of n, where n is the number of perturbation frequencies used simultaneously. Further, by using many frequencies the optimal frequencies may be included in the waveform. This multiplex advantage can complement the very different multiplex advantage arising in the spectroscopic portion of the measurement. The multiplex advantage of a particular spectroscopic instrument may accrue simultaneously with a multiplex advantage in the characterization of the time dependence of the sample response. Two extreme cases of the multifrequency waveform are suggested. One is a pulse waveform containing all frequencies within the bandwidth of the pulse generating equipment. In this case, the component frequencies of the pulse have the same phase, hence adding together in the time-domain to produce a large excursion in amplitude. Such pulses are readily generated by electrical means. The second extreme case also uses all frequencies within the bandwidth of the pulse generating equipment, but such that each component has random phase. In this case, the components do not add coherently at any point in the time domain. Hence, the dynamic range of the waveform applied to the sample is smaller, allowing more optimal use of the mechanical range of motion of the modulation device, but slightly complicating the measured data.
It is accordingly an objective of the present invention to measure the reorientation rates of submolecular components, particularly of polymeric materials, using a spectrometer together with the method and device described. It is also an objective of the present invention to use mechanical perturbation to simplify spectral interpretation. It is also an objective of the present invention to increase the signal-to-noise ratio of such measurements by using a multiplex advantage.